In the production of beer, a warm water extract of barley malt, with or without other unmalted grains such as rice or corn, is boiled with hops, cooled, and then subjected to the fermentative action of yeast. The warm water used to extract the malt allows the action of several enzymes in the malt to hydrolyze the starch in the barley (and in the corn or rice) to fermentable sugar.
As a result of material contained in the ingredients, or the mashing process, or the fermentation process, or a combination of two or more of the above, malt beverages are subject to an oxidative change in flavor. The resulting flavor is variously described as papery, cardboard-like, stale, or just plain "oxidized". This flavor change typically takes place within 2-3 months of room temperature storage, and is gradual and continuing. Most American manufacturers of beer recall beer from the market if it is more than about 4 months from the packaging date. Although the oxygen in a bottle or can of beer is typically consumed by the beer within less than 24 hours after packaging, the noticeable presence of oxidized flavor generally does not appear until many weeks later.
The flavor of oxidized malt beverages generally is attributed to unsaturated aldehydes notably trans-2-nonenal, and related compounds. These compounds are thought to be derived from unsaturated fatty acids, which split and then lose two hydrogen atoms. This loss of hydrogen is a form of oxidation.
The present practice of delaying the staling of beer is to maintain a low level of air (or oxygen) in the packaged beer, storing the beer at cold temperatures, and/or by the addition of sulfites.
Modern beer-filling machines are designed to achieve very low air levels in the packaged product. Typically, the present practice is to evacuate the bottle before it is filled with beer, or to replace the air in the evacuated bottle with carbon dioxide before filling, and to cause overfoaming in the bottle to displace the head space gases with beer foam. All of these practices can produce air levels of less than 0.5 ml. per 12 oz. bottle. But even these low levels of air still allow beer to oxidize in 2-3 months.
Another technique for stabilizing beer against oxidation is to add sulfur dioxide, in the form of bisulfite, to the beer. However, in the United States, addition of sulfur dioxide is limited by law to less than 10 ppm, and even those low levels product undesirable and sulfury aromas in some beers. Also the bisulfite, which works by binding to aldehydes, has many other aldehydes in beer to bind to (notably acetaldehyde, a normal by-product of fermentation), and so its action is often muted. Other countries such as Germany prohibit any addition of sulfur dioxide.